Blood Relationships:
Pointing to a boy in the photograph Reena said, "He is the only son of the only child of my grandfather." How Reena is related to that boy? Mother
Sister
Aunt
Cannot be determined
Explanation:
The boy in the photograph is the only son of Reenas grandfathers only son; i.e., the boy is the only son of Reenas father.
Hence, the boy is the brother of Reena or Reena is the sister of the boy.